Fuel rods of pressurized water nuclear reactors consist of pellets of a nuclear fuel material in oxide form stacked inside a tubular cladding, generally made of a zirconium alloy. The claddings of fuel rods must hold the nuclear fuel pellets and they constitute the primary barrier preventing the nuclear fuel from spreading inside the reactor.
In the case of local piercing of the cladding of a fuel rod, the reactor cooling water penetrates inside the cladding of the fuel rod; this water, which boils, reacts with the metal of the cladding to form hydrogen that produces hydridation of the metal of the cladding. This may result in further perforations of the cladding which amplify the results of the initial piercing.
It is therefore necessary to reduce to a minimum the risks of piercing of the fuel rod claddings, in particular at the bottom grid of the assemblies, where the rods are subjected to intense transverse hydraulic flows and are therefore susceptible to wear, because of the "fretting" phenomenon, by vibration against the rod supports constituted by the springs and the dimples.
Furthermore, the bottom grid of a nuclear fuel assembly is a location highly susceptible to trapping of debris which is vibrated by the circulating cooling fluid. This vibrating debris can damage the rods by friction and wear.
It has therefore been proposed to protect the fuel rod claddings by a layer of wear-resistant coating, in particular in the zones of the cladding which are most exposed to wear.
It has, for example, been proposed to improve the wear resistance of the fuel rod claddings made of zirconium alloy by depositing a hard coating on the external surface of the claddings, in particular by zirconium oxide (zirconia) deposits, these deposits being capable of guaranteeing some degree of protection against wear by the debris carried by the reactor cooling water.
In FR-A-89-12920, filed by FRAMATOME and COGEMA, a process was proposed for surface oxidation of a part made of a passivatable metal such as zirconium or a zirconium alloy, which makes it possible to produce a diffused layer of zirconium oxide on this part. This process is applicable in particular to coating the external surface of fuel rod claddings for a nuclear reactor.
In the context of this process for surface oxidation of a passivatable metal part, a gas activated by a cold plasma is used, i.e., a gas containing species activated by supply of energy. Using an activated gas makes it possible to obtain an oxide layer composed of fine and homogeneous grains at the surface of the part at moderate temperatures.
This process makes it possible to obtain oxide coating layers which guarantee a high degree of protection against the corrosion of elements such as cladding tubes; however, the layers obtained, being thin, do not guarantee very efficient protection against damage of mechanical origin, due, for example, to friction of debris against the cladding tubes.
It has also been proposed in WO-A-92 09716 to reduce the wear by fretting of zirconium alloy cladding tubes by producing carbide or oxide coatings on the surface of the cladding tubes, by using a gaseous phase or molten salts at a temperature of less than 500.degree. C. It has thus been possible to form protective layers whose thickness may be up to 2 .mu.m. The layers formed on the surface of the cladding tubes are still too thin to make it possible to guarantee very effective protection against mechanical damage, for example to prevent scratching of the surface of the cladding tubes.
It has also been proposed in U.S. Pat. No. 5,171,520 to produce a local coating of the nuclear fuel rods in those zones most greatly subjected to wear, by spraying ceramic and glass onto the surface to be coated. It is possible in this way to obtain layers with a thickness greater than 100 .mu.m made of materials such as zircon bound by aluminosilicates, borosilicates or calcioborates.
It has also been proposed in U.S. Pat. No. 5,227,129 to produce the coating for protection against wear and against corrosion from zircon nitrite, on the external surface of the cladding tubes of fuel rods, by an ionic deposition process.
Although the methods of protection by depositing a coating allow a substantial increase in the wear resistance of the cladding tubes of fuel rods, the coatings obtained have the drawback of being layers formed by diffusion of an element, so that the thickness of these layers is limited and high stress gradients are produced in the coated surface layers. If coatings with substantial thickness are produced, these coatings may exhibit defects which lead to corrosion-sensitization of the metal of the substrate in proximity to these defects.
Furthermore, the coating processes which can be used in the case of fuel rods including a zirconium alloy cladding tube must be carried out at a temperature such that the metallographic state of the cladding tube is not altered by the treatment.
In the case of a zirconium alloy which has undergone stress-relieving heat treatment, the treatments must be carried out at a temperature of less than 480.degree. C.
In the case of a zirconium alloy in the recrystallized state, the treatment must be carried out at a temperature of less than 650.degree. C.
Gas-phase chemical coating processes, generally called CVD (chemical vapor deposition) processes, are known which make it possible to produce deposits of different types on a substrate. In order to implement this process, a reactive gas, for example an oxidizing or carburizing gas, is caused to react, at the surface of the substrate to be coated, on a volatile metallic element component, in order to form in vapor phase a compound of the metallic element such as an oxide or carbide which is deposited on the substrate to be coated.
In particular, the metallic element may be the basic metallic element of the material constituting the substrate.
In the case of a zirconium alloy cladding tube, it is possible to deposit, for example, a compound constituted by a zirconium oxide on the zirconium-based alloy substrate.
In order to deposit the coating on the substrate, it is necessary to heat the substrate to a sufficiently high temperature, of the order of 700 to 1100.degree. C.
In the case of a zirconium alloy cladding tube, the temperature of deposition by the CVD process must not exceed the limits fixed by the metallographic structure of the alloy.
CVD deposition processes are known, in which a part of the energy necessary for activating the reaction is supplied by excitation of the reactive gas by a cold plasma. Excitation of the reactive gas makes it possible to create activated chemical species within the gas, which thus has an increased reactivity.
It is thus possible to lower the reaction temperature, which is entirely favorable in the case of certain applications.
Furthermore, there is no known method making it possible to produce a highly adherent coating of a metal oxide layer on a substrate constituted principally by a metal sensitive to attack by halides and hydrogen.